U.S. patent number 5,486,406 [Application Number 08/334,999] was granted by the patent office on 1996-01-23 for green-emitting organometallic complexes for use in light emitting devices.
This patent grant is currently assigned to Motorola. Invention is credited to Song Q. Shi.
United States Patent |
5,486,406 |
Shi |
January 23, 1996 |
Green-emitting organometallic complexes for use in light emitting
devices
Abstract
A new class of organometallic complexes for use in
electroluminescent (EL) devices and a method of preparation are
disclosed. The organometallic complexes are prepared by mixing
organic ligands with metal salts in the presence of a base and a
layer is formed in an EL device by vacuum evaporation. The
organometallic material in the EL device serves as either an
electron transporting layer or a light emission layer, or both.
Inventors: |
Shi; Song Q. (Phoenix, AZ) |
Assignee: |
Motorola (Schaumburg,
IL)
|
Family
ID: |
23309794 |
Appl.
No.: |
08/334,999 |
Filed: |
November 7, 1994 |
Current U.S.
Class: |
428/209; 428/457;
313/504; 252/301.16; 252/301.22; 428/917 |
Current CPC
Class: |
H05B
33/14 (20130101); C09K 11/06 (20130101); Y10T
428/24917 (20150115); Y10T 428/31678 (20150401); Y10S
428/917 (20130101) |
Current International
Class: |
C09K
11/06 (20060101); H05B 33/14 (20060101); B32B
009/00 () |
Field of
Search: |
;428/457,209,690,917
;313/504 ;252/301.16,301.22 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Junji Kudo et al., 1,2,4-Triazole Derivative as an Electron
Transport Layer in Organic Electroluminescent Devices, Jpn. J.
Appl. Phys. vol. 32 (1993) pp. L917-L920, Part 2, No. 7A, 1 Jul.
1993..
|
Primary Examiner: Ryan; Patrick J.
Assistant Examiner: Jewik; Patrick R.
Attorney, Agent or Firm: Parsons; Eugene A.
Claims
What is claimed is:
1. An organic light emitting device comprising:
a first conductive layer;
a layer of first carrier transporting and second carrier blocking
material positioned on the first conductive layer;
a layer of organometallic material positioned on the layer of first
carrier transporting and second carrier blocking material and
having the following general formula: ##STR4## where: M.sup.2 is a
divalent metal ion, and
R.sup.1 to R.sup.8 represent substitution possibilities at each
position and each represent hydrogen or hydrocarbon groups or
functional groups;
a layer of second carrier transporting and first carrier blocking
material positioned on the layer of organometallic material;
and
a second conductive layer positioned on the layer of second carrier
transporting and first carrier blocking materials.
2. An organic light emitting device as claimed in claim 1 wherein
the first carriers are holes and the second carriers are
electrons.
3. An organic light emitting device as claimed in claim 1 wherein
the first conductive layer is p conductivity and the second
conductive layer is n conductivity.
4. An organic light emitting device as claimed in claim 1 wherein
one of the first and second conductive layers are transparent to
light emitted by the organometallic emissive layer.
5. An organic light emitting device comprising:
a glass substrate having a substantially planar surface;
a first conductive transparent layer positioned on the glass
substrate
a layer of hole transporting and electron blocking material
positioned on the first conductive layer;
a layer of organic active emitter material deposited on the hole
transporting and electron blocking layer;
a layer of organometallic complex material positioned on the layer
of organic active emitter material as electron transporting and
hole blocking material, the organometallic complex material having
the general formula: ##STR5## where: M.sup.2 is a divalent metal
ion, and
R.sup.1 to R.sup.8 represent substitution possibilities at each
position and each represent hydrogen or hydrocarbon groups or
functional groups; and
a second conductive layer positioned on the layer of organometallic
complex material.
Description
FIELD OF THE INVENTION
This invention relates to organic electroluminescent materials used
in devices such as light emitting diodes.
BACKGROUND OF THE INVENTION
Organic electroluminescent (EL) devices are ideal candidates for
use in portable display applications because of their low power
drain and capability of a full range of colors.
A typical device consists of thin layers of organic molecules
sandwiched between transparent and metallic electrodes. Under an
applied bias, oppositely charged carriers are injected from the
opposing contacts and are driven through the device by the electric
field. Some of these oppositely charged carriers capture one
another within the emissive layer to give out light at a wavelength
corresponding to the energy gap of the organic emissive materials.
In order to achieve high EL efficiency, it is necessary to balance
the rates of injection of electrons and holes from opposite
contacts into the device. In most case, the electron injection has
proved to be more difficult then hole injection because of the
relative large energy barrier existing at the n-contact and organic
interface. To lower the energy barrier for efficient electron
injection, often metals with low work function such as calcium,
magnesium, etc. are needed as the electron-injecting contact. An
alternative way to lower the energy barrier for efficient electron
injection is to use an organic material of high electron affinity
at the metal-organic interface. An organic material of high
electron affinity has low "Lowest-Unoccupied-Molecular-Orbit"
(LUMO) energy level that reduces the energy barrier for electron
injection at the metal-organic interface, thus increasing the
electron injection rate, resulting in a device of high efficiency
and low working voltage.
In the prior art, a class of organic materials that have exhibited
high EL efficiency in devices, are those based on metal complexes
of 8-hydroxyquinoline and its derivatives (Vanslyke et al U.S. Pat.
Nos. 4,539,507; 5,150,006). Another class of organometallic
complexes that have also resulted in highly efficient organic EL
devices is disclosed in a copending U.S. Patent Application
entitled "NEW ORGANOMETALLIC COMPLEXES FOR USE IN LIGHT EMITTING
DEVICES", filed 12 Sep. 1994, bearing Ser. No. 08/304,451, and
assigned to the same assignee.
It is a purpose of this invention to provide a class of new
organometallic complexes with high electron affinities for use in
light emitting devices.
It is another purpose of the present invention to provide a class
of new organometallic complexes for emission in the green range in
light emitting devices.
It is a further purpose of this invention to provide preparation
methods for the disclosed organometallic complexes for use in light
emitting devices.
SUMMARY OF THE INVENTION
The above problems and others are at least partially solved and the
above purposes and others are realized in a new class of
organometallic complexes having the following general formula:
##STR1## where: M.sup.2 is a divalent metal; and
R.sup.1 to R.sup.8 represent substitution possibilities at each
position and each represents hydrogen or hydrocarbon groups or
functional groups.
In addition, the preparation of the new class of organometallic
complexes is novel and the complexes are utilized as either an
electron transporting layer or an active emissive layer, or both in
organic electroluminescent devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings:
FIG. 1 is a simplified sectional view of an organic EL device in
accordance with the present invention; and
FIG. 2 illustrates a schematic energy-level diagram for a single
layer organic EL device under forward bias.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to a class of new organometallic
complexes for use in organic light emitting devices which, in
general, consist of thin layers of organic molecules sandwiched
between transparent and metallic electrodes.
FIG. 1 illustrates a simplified cross-sectional view of one
embodiment of an organic EL device 10 incorporating the present
invention. Organic EL device 10 includes a substrate 11 which in
this specific embodiment is a glass plate having a relatively
planar upper surface. An electrically conductive layer 12 is
deposited on the planar surface of substrate 11 so as to form a
relatively uniform electrical contact. A first organic layer 13 of
hole transporting material is deposited on the surface of
conductive layer 12. A second organic layer 14 of emissive material
is deposited onto first organic layer 13. Then a third organic
layer 15 of electron transporting material is deposited on the
surface of layer 14 and a second electrically conductive layer 16
is deposited on the upper surface of third organic layer 15 to form
a second electrical contact.
While it should be understood that light generated within second
organic layer 14 can be emitted either through first organic layer
13, conductive layer 12 and substrate 11 or through third organic
layer 15 and second conductive layer 16, in the present embodiment,
substrate 11 is formed of glass and conductive layer 12 is formed
of organic or inorganic conductors, such as conductive polyaniline
(PANI), indium-tin-oxide (ITO), which are substantially transparent
to visible light so that the emitted light exits downwardly through
substrate 11 in FIG. 1.
Further, in this embodiment, conductive layer 16 is formed of any
of a wide range of metals or alloys in which at least one metal has
a work function less than 4.0 eV. By the proper selection of
material for conductive layer 16, the work functions of the
materials making up layers 15 and 16 are substantially matched to
reduce the required operating voltage and improve the efficiency of
organic LED 10. Additional information on work function matching is
disclosed in a copending U.S. Patent Application entitled "Organic
LED with Improved Efficiency", filed 12 Sep. 1994, bearing Ser. No.
08/304,454, and assigned to the same assignee.
Also, in FIG. 1 organic EL device 10 has a potential applied
between layers 12 and 16 by means of a potential source 17. In this
embodiment conductive layer 12 is a p-type contact and conductive
layer 16 is an n-type contact. The negative terminal of potential
source 17 is connected to conductive layer 16 and the positive
terminal is connected to conductive layer 12. Electrons injected
from the n-type contact (layer 16) are transported through organic
layer 15 and into organic layer 14 (the emissive layer). Holes
injected from the p-type contact (layer 12) are transported through
organic layer 13 and into organic layer 14 (the emissive layer),
where upon an electron and a hole recombination a photon is
emitted.
Organic layer 13 includes any known hole transporting organic
molecules, such as aromatic tertiary amines (U.S. Pat. No.
5,150,006) and/or hole transporting polymers such as poly(phenylene
vinylene), and is used to transport holes into organic layer 14 and
confine electrons in organic layer 14. Organic layer 15, includes
any known electron transporting materials, such as
tris(8-hydroxyquinolino)aluminum (U.S. Pat. No. 4,539,507) and the
complexes disclosed in the present invention. Organic layer 15 is
used to transport electrons into organic layer 14 and confine holes
within organic layer 14. Thus the holes and electrons have a
maximum opportunity to recombine in organic layer 14 to give off
light.
In general, the complexes disclosed in the present invention are
utilized as either an electron transporting layer or an active
emissive layer, or both in organic electroluminescent devices. When
the disclosed complexes are used only as an electron transporting
material of layer 15, an additional emissive material is needed to
form layer 14. When the disclosed complexes are used only as the
emissive material of layer 14, an additional electron transporting
material is needed to form layer 15. When the disclosed complexes
are used as both emissive and electron transporting material,
layers 14 and 15 are generally combined into one layer.
In accordance with the present invention, organic layer 14 (the
emissive layer) and/or 15 (the electron transporting layer) in
organic EL device 10 are formed of at least one organometallic
complex having a general formula as shown in the following:
##STR2## where: M.sup.2 is a divalent metal ion; and
R.sup.1 to R.sup.8 represent substitution possibilities at each
position and each represent hydrogen or hydrocarbon groups or
functional groups such as cyano, halogen, haloalkyl, haloalkoxy,
alkoxyl, amido, amino, sulfonyl, carbonyl, carbonyloxy and
oxycarbonyl etc.
The above complexes are generally prepared via the following
reaction: ##STR3## where: M.sup.2 is a divalent metal ion;
X is an anionic group including halides, sulfate, nitrate,
etc.;
n=1 or 2; and
R.sup.1 to R.sup.8 represent substitution possibilities at each
position and each represent hydrogen or hydrocarbon groups or
functional groups.
In a typical reaction, ligand L is suspended in an alcoholic
solvent such as methanol or ethanol, and is treated with one
equivalent amount of base such as sodium hydroxide, sodium
ethoxide, etc. under an inert atmosphere. After a homogeneous
solution has been attained, 1/2 of an equivalent amount of divalent
metal salt (MX.sub.n, n=1 or 2) is added into the solution. The
precipitation that forms is collected by filtration and further
purified by sublimation.
It is well known from organic chemistry that ligand L, a triazole
derivative, is an electron deficient system that has a high
electron affinity. In the prior art, triazole derivatives have been
used by Kido and coworker as an electron transporting layer in
organic EL devices (Jpn. J. Appl. Phys. 1993,32, L917.). The
complexes of triazole derivative L with metal ions have even higher
electron affinity. They can be used as either an electron
transporting layer or an active emissive layer, or both in organic
electroluminescent devices.
FIG. 2 illustrates a schematic energy-level diagram for a single
layer organic EL device under forward bias. Line 110 represents the
vacuum level, lines 130 and 150 represent the Fermi levels of metal
and ITO layers or contacts, and lines 120 and 140 represent the
LUMO and HOMO of an organic complex. The .phi..sub.1 and
.phi..sub.2 energy levels are the work functions of the ITO and
metal contacts, while the A1 energy level is the electron affinity
of the organic complex physically positioned between the ITO and
metal contacts. The high electron affinity (A.sub.1) means a low
LUMO energy level that, in turn, reduces for electron injection the
energy barrier (.phi.2--A1) between the n-type metal contact and
the organic complex. The efficient electron injection produced by
lowering the energy barrier, translates into higher luminescent
efficiency and lower operating voltage in the organic EL
devices.
Of the various ligands that satisfy the requirements of the
invention, 2-(2'-hydroxy-5'-methylphenyl)benzo-triazole (TP) is the
simplest and most commercially available material. TP forms
complexes with many divalent metal ions such as Be.sup.2+,
Mg.sup.2+, Zn.sup.2+ to yield Be(Tp).sub.2, Mg(Tp).sub.2 and
Zn(Tp).sub.2. These complexes are green fluorescent upon photo- or
electro-excitation.
Organic layer 14 (the emissive layer) in organic EL device 10 (FIG.
1) is commonly deposited by thermal vapor deposition, electron beam
evaporation, chemical deposition or the like. The emission peaks of
the above embodied organometallic complexes when utilized in
organic LEDs range from 510 nm to 560 nm, which are in the regions
of green to greenish-yellow on the CIE 1931 chromaticity
diagram.
EXAMPLES
This invention will be further described by the following examples,
which are intended to illustrate specific embodiments of the
invention but not to limit its scope.
Example 1
The following procedures for synthesis of Be(Tp).sub.2 can be used
to prepare all the divalent metal complexes disclosed in this
invention, except that metal chloride or nitrate salt, instead of
metal sulfate salt, is used in some cases depending on the
availability of the salts.
Procedure One
A mixture of 20 mmol of Tp (Ciba Geigy Company) in 80 mL of
methanol is treated with 20 mmol of sodium hydroxide pellet (Fisher
Scientific Company) under argon atmosphere. The mixture is stirred
until all of the sodium hydroxide pellets are dissolved. The
mixture is then added with 10 mmol of beryllium sulfate
tetrahydrate (Aldrich Chemical Company). The resulting mixture is
stirred at reflex for 16 hours and allowed to cool to room
temperature. The yellow fluorescent solid is collected by
filtration, washed with methanol, and dried under vacuum to afford
Be(Tp).sub.2 in 78% yield.
Procedure Two
A solution of 12 mmol of sodium hydroxide (Fisher Scientific
Company) in 60 mL of de-ionized water/methanol (1:1) mixture is
added with 12 mmol of Tp (Ciba Geigy Company) under an inert
atmosphere. The reaction mixture is warmed up with a water bath and
stirred until a homogeneous solution is attained. To the solution
is added dropwise through an addition funnel a solution of 6 mmol
of beryllium sulfate tetrahydrate (Aldrich Chemical Company) in 20
mL of de-ionized water. The yellow fluorescent precipitation which
results is filtered and washed with de-ionized water and methanol
to yield Be(Tp).sub.2 in 85% yield after drying.
Example 2
The following procedure is used for the purification and
characterization of the organometallic complexes produced and
disclosed above.
The solid complex to be purified is placed into the sealed end of
an one-end-sealed quartz tube which has been divided into several
zones that are connected together with ground joints. The quartz
tube is then inserted into a one-ended-sealed Pyrex tube which has
been connected to a vacuum system. The sealed end of the quartz
tube is in contact with the sealed end of the Pyrex tube. The Pyrex
tube is then evacuated to 10.sup.-6 torr with a diffusion pump and
the sealed end of the Pyrex tube is heated in a tube furnace. The
pure product is sublimed into different zones than volatile
impurities in the quartz tube and purification thus is achieved.
The sublimation temperature ranges from 250.degree. C. to
350.degree. C. depending on the complexes.
In general, the complex prepared from procedure one described in
Example 1 offers better overall yields then procedure two after
sublimation.
The pure complexes are analyzed and characterized by
ultraviolet-visible, infrared, photoluminescence spectra as well as
elemental analyses. This provides confirmation for the structures
and compositions of the desired complexes.
Thus, a class of new organometallic complexes for use in light
emitting devices has been disclosed, along with preparation methods
for the disclosed organometallic complexes and methods of
fabrication of light emitting devices. The new organometallic
complexes have been utilized as either an electron transporting
layer or an active emissive layer, or both in organic EL
devices.
While I have shown and described specific embodiments of the
present invention, further modifications and improvements will
occur to those skilled in the art. I desire it to be understood,
therefore, that this invention is not limited to the particular
forms shown and I intend in the appended claims to cover all
modifications that do not depart from the spirit and scope of this
invention.
* * * * *